Bass Traps

If you don't know why bass traps are required, please read the Why Bass Traps? page.

Rather than buy commercially made bass traps, I prefer to make my own.  It’s not only a rewarding experience but very cost effective too.  Bass traps don’t have to boring nondescript black and white slabs either; set your imagination free, express your creativity!

Bass traps are most effective when straddled across the room’s tri-corners.  A tri-corner is the three surface junction of wall/wall/ceiling or wall/wall/floor.  It is often not practical to fill all eight tri-corners; filling as many as possible will make a substantial difference though.    How big should they be?  Think in terms of: what's the largest I can make ‘em and still get in the room rather than what’s the smallest I can get away with.

If you'd like to calculate the modal frequencies of your room, feel free to download a copy of my Room Modes Calculator spreadsheet (462kb)

Instructions: to calculate your room's first 999 modes (Axial, Tangential and Oblique) enter the Length, Width and Height in metres in the spreadsheet; it also includes a handy imperial/metric converter.  Also calculated are: room ratio, cubic volume, wall and floor areas.

If you are considering building a new room, then feel free to download a copy of my Bolt’s Ratios spreadsheet (176kb)

Instructions; by entering just the height (in metres) of a proposed room in cell H2, this spreadsheet calculates: room width and length, cubic capacity, total wall area, total floor and ceiling area for four Bolt’s ratios; results displayed in both metric and imperial.  The right hand side shows the mode type, number, frequency and distribution of 999 modes for each of the four ratios.  Page two contains various imperial and metric converters.  

PVA glue (white glue for woodwork) was used to stick two 5cm slabs of 75kg/m³ mineral wool together.  The resulting 10cm slab was then stuck to a 2.5cm sheet of rigid Styrofoam with 2cm thick wooden strips (also glued with PVA) for stapling the cloth to and fixing the mounting eyebolts.

The finished product (one of four) mounted in the wall/ceiling junction:

In a thread on the Sound On Sound forum some concerns were expressed about these traps, hence the reason for the following:   

From my tests, I’ve found that with 3cm thick Styrofoam (Dow Chemicals Wallmate), absorption only starts from somewhere just over 1kHz.  There is absolutely no effect on frequencies lower than 1kHz; i.e., 3cm thick Styrofoam is acoustically transparent below 1kHz.   

10cm circular holes were cut in the 1cm thick MDF sides to reduce weight and increase the absorption surface area.   To dress the sides, strips of red cloth were stapled to the inner edges.

Thin chain was fixed to each corner and a single eyebolt fixing used to secure the unit to the wall.  The unit was then filled with 15cm of 75kg/m³ mineral wool.  Leftover cloth strips were stapled across the front to hold the mineral wool in place and a yellow calico covered wooden frame fitted to the front to finish off the job.  A pair were made and fitted on opposite wall/ceiling junctions to maintain the left/right mirror image control room symmetry.

Because the control room ceiling is slopped (rising at the rear) and you are only seeing a portion of it, the traps may not appear square. In real life they are of course perfectly square.

The blue covered boxes in the background are vertical corner traps fitted from floor to ceiling and filled with mineral wool triangles occupying the four available control room tri-corners; floor/walls and ceiling/walls.    

The lower middle section wasn't filled with mineral wool (it made very little difference to performance) and the resulting extra shelf space was a welcome addition.  Again, to maintain symmetry, a mirror image unit is fitted to the left corner.

Varying sized slots cut in 2cm thick front panels

Dressed with screen printed material made by a friend

At the time of construction it wasn't practical to measure their performance but according to calculations, they should be responding to a range of frequencies from around 200 to 500Hz.  I do have my doubts as to their effectiveness but, combined with all the other treatments in the room, I'm very happy with the results.

An airtight 18cm deep box lined with 15cm of 75kg/m³ mineral wool leaving an air gap at the front for the 3mm plywood front panel (not shown) to freely vibrate.  The cloth covered larger front helps conceal the ugliness of the box.

Two resonant panel traps were also built in under the main monitors in the control room.  The same 3mm ply was used for the fronts and although they have much larger volume (the void under the monitors), the front resonant panels are considerably smaller.  Testing revealed the control room resonant traps to have a very narrow and well defined centre frequency of around 48Hz whereas the larger panels in the live room exhibit a much broader centre frequency of around 40 to 100Hz. 

Mineral Wool

If you are looking for mineral wool details and suppliers, I have a  Mineral Wool page or, alternatively, check out the following links:

© Tim Rainey 2008

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 Feedback, questions and comments welcome

Last updated:  2 January 2010

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